The ATLAS experiment, on the LHC at CERN(Geneva), requires a great deal of offline processing of the data, as well as Monte Carlo simulation. This is currently provided by a Grid of large computing centres and Universities around the world, and ATLAS typically fills 100,000 cores. It has become apparent that computing resources will increasingly be multi-core, and that the current operational model, of N serial jobs per node, is becoming unsustainable. In particular, the memory footprint per core can be reduced by running a single job on multiple cores. ATLAS has developed AthenaMP for this purpose, and it is currently under test.